It is well known in the art that the photochemical formation of carboxylic acids, be it by amplified means such as a catalytic acidic-decomposition of a tertiary butyl ester, or as in the case of the photochemical decomposition of 1,2-naphthoquinone diazides, can be employed to produce high resolution and high efficiency resists. This type of reaction is being relied upon extensively in the production of positive working resists.
The manufacture of integrated circuits and other patterned devices relies primarily on resist materials that enable the formation of high resolution patterns. In the search for materials and methods for formation of patterns below 0.25 microns, it is recognized that such patterns require exposure sources based on UV radiation below 248 nm, or on x-ray, or on e-beams. Likewise, it is essential to employ resist materials suitable for use with short wavelengths sources. In the case of UV radiation, it might be convenient to use excimer laser sources that produce radiation at 193 nm.
An example of a positive resist suitable for short wavelength (193 nm) applications is the amplified resist described in the Journal of Vacuum Science and Technology B9 (6), p3357 (1991), by R. D. Allen et al., and also in U.S. Pat. No. 5,071,730, which describes a terpolymer based on a methacrylate backbone containing tertiary butyl esters, methyl esters and unesterified carboxylic acids side chains. Resist formulations also contain photoacid generating groups that decompose during the irradiation and cause a catalytic decomposition of the tertiary butyl esters groups, resulting in additional free carboxylic acid groups in imaged areas. These can be dissolved faster than the unexposed polymer with dilute aqueous bases. However, due to the existence of free carboxylic acid groups in non-imaged areas, the selectivity of the development is somewhat compromised.
U.S. Pat. No. 5,212,047 by Walter R. Hertler et al., assigned to E. I. Du Pont de Nemours and Co. describes another amplified resist material that provides excellent resolution and sensitivity. The composition involves polymers having recurring pendant acid labile .alpha.-alkoxyalkyl carboxylic acid ester moieties in the presence of an acid generator activated by UV, visible, x-ray or e-beam radiation. The mode of operation of this resist is similar to other amplified resists that use acid labile groups and is well known in the art.
It is well known in the art that poly(methyl methacrylate) (PMMA) undergoes main chain scission with a low quantum yield of 0.04 to 0.14, depending on the intensity and wavelength of the radiation. Side chain scission with a minute quantum yield of 10.sup.-6 has also been reported, but is too inefficient for practical resist applications. The reported photochemistry of PMMA and its analogues, leads to solvent developable positive images and relatively low efficiency lithography. (Wayne M. Moreau, Semiconductor Lithography, p 59, Plenum Press, NY 1988).
Japanese Laid-Open Patent Application (Kodai) No. 1-244447, Sep. 28, 1989 by Nakase et al., assigned to Toshiba Corp., suggests that films of poly(2-hydroxyethyl methacrylate) and poly(2-hydroxypropyl methacrylate) can be used for pattern formation, particularly with e-beams, and developed with isopropyl acetate, an organic solvent. The mode of development of patterns therein suggests that the scission products are expected to behave in the normal mode attributable to the majority of poly acrylic and methacrylic esters, namely main chain scission.
C. F. Vernon et al. reported (Polymer International, 27, 243 (1992)), that poly(2-hydroxyethyl methacrylate) undergoes ablation when exposed with excimer laser radiation at 193 nm. They suggested that at high radiation fluxes (&gt;280 mJ/sq.cm.) main chain scission occurs while at low fluxes, scission of the entire side chain (2-hydroxyethoxycarbonyl) is obtained, with possible production of carbon dioxide.
U.S. Pat. No. 3,130,047 suggests that diazoquinone groups of sulfonyl esters of naphthoquinone-(1,2)-diazides can be converted photochemically to carboxylic acids that are soluble in aqueous alkali which in turn can cause solubilization of resins that contain these molecules. Suitable developers for the resists described therein include aqueous solutions of alkali metals or non-metallic bases such as tetramethyl ammonium hydroxide.